Metallic thin films with roughness of nano-scale are of great use and importance in the fields of energy storage materials, catalysts, and sensors. In spite of their importance, however, only a few methods have been suggested to fabricate such nanoporous metal films. The examples include chemical/electrochemical deposition in a liquid crystalline template or dealloying of alloys. U.S. Pat. No. 6,203,925 discloses a method for the fabrication of a porous metal electrode having a substantially regular structure and uniform pore size. The method disclosed in the U.S. Pat. No. 6,203,925 comprises reducing a mixture including a source of a metal; a solvent; and a structure-directing agent present in an amount sufficient to form a liquid crystalline phase in the mixture, to form a metal-organic composite, and removing the organic from the composite. Another exemplarily method for the fabrication of the mesoporous metal electrode is disclosed in U.S. Pat. No. 6,503,382. The method disclosed in the U.S. Pat. No. 6,503,382 comprises electrodepositing a metal-organic composite from a mixture onto a substrate to form a porous film, wherein the mixture comprises a source of a metal, a solvent, a structure-directing agent in an amount sufficient to form a homogeneous lyotropic liquid crystalline phase in the mixture, and removing the organic from the metal-organic composite. The method has an advantage to produce mesoporous metal electrode with a regular structure and uniform pore size. However, the methods suffered from the disadvantage that they could be applicable only to the liquid crystalline phase such as lamellar (Lα), hexagonal (H1), and cubic (V1) phase, preferably to H1 or V1 phase. Mixtures having the liquid crystalline phase have very high viscosity. As thus, highly viscous residues remain on the substrate even after the substrate onto which the metal-organic composite was already formed is lift off from the liquid crystalline mixture. As a result, physical removing of the residues should be performed before subjecting the substrate to the step of removing the organic from the metal-organic composite. The removal of the residues is a manually-performed, cumbersome work and prohibits automation of the manufacturing processes. Moreover, the method requires highly concentrated metal source. According to U.S. Pat. Nos. 6,203,925 and 6,503,382 mentioned in the above, the concentration of the metal source reaches up to 29 wt %, based on the mixture of the solvent and the structure-directing agent (Langmuir, 1998, 14, 7340-7342, Attard et al.). However, excessive use of the metal source makes the method to be ineffective and expensive, and deteriorates the disadvantages caused by the residing of highly viscous residues on the substrate. Moreover, the temperature must be kept as low as possible in order to maintain the mixture in a liquid crystalline phase. According to the phase study by Attard et al. (Langmuir, 1998, 14, 7340-7342), octaethyleneglycol monododectylether (C12EO8, in which EO represents ethyleneoxide), which is a representative structure-directing agent, does give a liquid crystalline mixture at above 85° C. In addition, octylphenoxy-polyethoxy ethanol (Triton X-100™) gives a liquid crystalline mixture only at a temperature of no more than 20-30° C. (Langmuir 2000, 16, 4922-4928, Galatanu et al.). The addition of the metal source further interferes with the formation of the liquid crystalline phase. As a result, the methods disclosed in the above two U.S. patents have a limit in terms of the temperature. Low temperature retards the electroplating rate, and so it takes long time to fabricate the mesoporous metal electrode. In addition, some structure-directing agents due to the limitation of the temperature are not applicable to the methods.
Stucky et al. (Advanced Materials, 2003, 15, 2018) showed that the potential-controlled-surfactant-assembly method worked in the solution of low surfactant concentration around critical micelle concentration (cmc). According to their report, cylindrical/hemicylindrical micelle assembly was formed below cmc on electrode/solution interface by electric field and functioned as a template for nanoporous platinum thin film with pore size of ˜4 nm and wall thickness of ˜4 nm. In these cases, 100 nm-thick platinum films could be formed within 30 s and the problems encountered in the liquid-crystalline-template method seem to be relieved. However, the electrical field at the surface substantially affects the morphology of the plated platinum in this method. Nanoporous platinum was deposited only near 0.2 V versus Ag/AgCl (4 M KCl), where cylindrical/hemicylindrical assembly was produced. In the other potential range, bilayer assembly appeared and inhibited the nanopore formation.